This proposal aims to examine, for the first time, the correlation between endothelial ultrastructural differentiation and macromolecular permselectivity during normal angiogenesis. In particular, the role of biomolecular differentiation of the endothelial glycocalyx in modulating the access of macromolecules to the junctional clefts and plasmalemmal vesicles, during their concomitant ultrastructural differentiation, will be evaluated. The extraembryonic microcirculation of the chick chorioallantoic membrane (CAM), which requires neither anesthesia nor surgical manipulation for direct microscopic observation, will serve as models of angiogenesis. Evaluations of precapillary, capillary, and postcapillary segments at days 6, 10, 14, and 18 of embryogenesis will be approached, in part, by intravital fluorescence microscopy and the integrated optical intensity technique to assess differential rates of extravasation of a series of graded molecular weight FITC-dextrans. The measured rates will provide an index of differentiation of endothelial permselectivity during normal angiogenesis. Ultrastructural morphometric detection of these same dextrans by colloidal gold immunocytochemistry will serve to correlate differentiation of endothelial plasmalemmal vesicles and junctional clefts with macromolecular permselectivity. Ultrastructural profiles of specific lectins and charged markers within the endothelial glycocalyx during the stages of endothelial cytodifferentiation will complete the analysis of the paracellular and vesicular pathways. Thus, the influence of biomolecular differentiation of the glucocalyx on the regulation of macromolecular access to these pathways will be evaluated. In addition, the effects of histamine on the glycoconjugate microdomains within the glycocalyx, and hence, on macromolecular transport will be tested. Taken as a whole, these data on normal angiogenesis will serve to provide the basis for understanding the derangements that occur during pathologic angiogenesis associated with wound healing and tumorigenesis.